1. Field of the Invention
The present invention relates generally to an electronic circuit coupled to an antenna for impedance matching, and more specifically to such a circuit for impedance matching between an antenna and a feeder at two resonant frequencies via which two-way communications are established. The present invention is well suited for an antenna which is mounted on a portable radio telephone unit and thus required to be small in size.
2. Description of the Related Art
In recent years, a large number of handheld portable radio telephone units have been widely used and this tendency is ever increasing. A portable radio telephone unit communicates with a base station via two channels and thus requires an antenna to resonate at two different frequencies.
Before turning to the present invention it is preferable to briefly discuss, with reference to FIG. 1, a known antenna which resonates at two different frequencies. This prior art is disclosed in Japanese. Laid-open Patent Application No. 63-286008 on Nov. 22, 1988.
As shown in FIG. 1, an antenna depicted by numeral 10 is provided with two antenna elements 12 and 14 which resonate respectively at different frequencies f.sub.1 and f.sub.2 (f.sub.1 &lt;f.sub.2). The element 12 has a rod-like configuration while the other element 14 takes the form of a coil. One end of the coil 14 is electrically coupled to the top of the element 12. In order to electrically insulate the two antenna elements 12 and 14, an insulator tube 16 is provided between the two elements 12 and 14.
The element 12 is .lambda..sub.1 /2 long (where .lambda..sub.1 is a wavelength of the resonant frequency f.sub.1). The coil 14 extends on the top portion of the element 12 over a distance (.lambda..sub.1 -.lambda..sub.2) and has a stretched length of .lambda..sub.2 /4 (where .lambda..sub.2 is a wavelength of the resonant frequency f.sub.2).
The antenna assembly 10 has one end coupled to an impedance matching circuit 17 which is in turn coupled, via a feeder 18, to a front end (not shown) of a handheld portable unit (not shown).
In order to achieve better portability, a handheld mobile unit antenna can be accommodated in the unit case when it is not used. However, It is difficult for the antenna of FIG. 1 to be contained in is not used in that the antenna 10 has a bulky top portion.